Oxidation of metal sulfides using thermotolerant bacteria

ABSTRACT

A process for recovering precious or base metals from particulate refractory sulfide materials comprises: a) contacting the sulfide material with an aqueous solution containing a thermotolerant bacteria culture capable of promoting oxidation of the sulfide material at a temperature in the range from 25° to 55° C., b) separating the oxidized residue from the aqueous liquid, and, c) treating the oxidized residue and/or the aqueous liquid to recover metal. In this context, a thermotolerant bacterium is one which has an optimum growth temperature of 40° to 45° C., and an operating temperature of 25° to 55° C.

DESCRIPTION

The present invention relates to a process for the treatment of metalcontaining materials by bacterial oxidation.

FIELD OF THE INVENTION

It is known that recovery of metals especially precious metals and basemetals from refractory sulphide materials can be enhanced by bacterialoxidation or leaching. The bacterial treatment subjects the sulphidematerial to a pre-oxidation. The refractory sulphide materials can takea wide variety of forms including mineral sulphides, carbonaceoussulphide ores, sulphide flotation concentrates, sulphide gravityconcentrates, sulphide tailings, sulphide mattes and sulphidic fume.

The precious metals and some base metals remain in the oxidised solidresidue and can be recovered by conventional carbon in pulp or otherchemical leaching processes. Some base metals such as copper, zinc andnickel go into solution and may be recovered directly by conventionalsolvent extraction and electrowinning.

In the past, bacterial oxidation of precious or base metal containingsulphide materials has typically been conducted using bacteria of theThiobacillus species. However, the Thiobacillus species can only operateat temperatures up to about 40° C. Further, the oxidation effected byThiobacillus bacteria is an exothermic reaction and it is sometimesnecessary to cool the process reactors to prevent the temperatureexceeding that at which the Thiobacillus bacteria can operate.

SUMMARY OF THE INVENTION

The present invention provides a process for the bacterial oxidation ofmetal containing sulphide materials using thermotolerant bacteria whichcan operate at higher temperatures than conventional Thiobacillusbacteria. In accordance with one aspect of the present invention thereis provided a process for recovering metals from particulate refractoryprecious or base metal containing sulphide materials which comprisescontacting the sulphide material with an aqueous solution containing athermotolerant bacteria culture (as herein defined) capable of promotingoxidation of the sulphide material at a temperature in the range from25° to 55° C., separating the oxidised residue from the aqueous liquidand treating the oxidised residue and/or the aqueous liquid to recovermetal therefrom.

DESCRIPTION OF THE INVENTION

The thermotolerant bacteria used in the present invention are asdescribed in "Thermophiles General, Molecular, and Applied Microbiology"edited by Thomas D. Brock and published by John Wiley & Sons (1986). InChapter 1 of this publication, there is illustrated in FIG. 1(b) a graphshowing that thermotolerant bacteria grow at temperatures lower thanthose preferred by moderate and obligate or extreme thermophiles.

In the context of the present invention, a thermotolerant bacteria isone which has an optimum growth temperature of 40° to 45° C. and anoperating temperature of 25° to 55° C. Preferably, the aqueous solutionused in the process of the present invention is acidic. It has beenfound that the optimum acidity of the aqueous liquid for growth of thethermotolerant bacteria culture used in the present invention is in therange from pH 1.3 to 2.0, whilst the optimum acidity of the aqueousliquid for operation of the process of the present invention is in therange from pH 0.5 to 2.5.

The bacterial oxidation step of the process of the present invention isconducted in the presence of nutrients which are typically dissolvedsalts of nitrogen, potassium and phosphorus. The nutrients may alreadybe present in the aqueous liquid or they may be added thereto. Thenutrient materials promote the growth of the thermotolerant bacteria.

It is preferred that the thermotolerant bacteria be acidophilic in viewof the pH conditions under which the process of the present invention ispreferably conducted. Further, the thermotolerant bacteria used in theprocess of the present invention are typically aerobic and thus theaqueous liquid is preferably aerated during the operation of the processto ensure that there is an adequate supply of oxygen for the bacteria.Still further, it is found that the thermotolerant bacteria culture usedin the process of the present invention is typically capable ofautotrophic growth. Yet further, the thermotolerant bacteria culturetypically does not require additional CO₂ over and above that normallyavailable from ambient air.

The thermotolerant bacteria culture used in the process of the presentinvention may be capable of oxidising arsenic (III) to arsenic (V) inacidic aqueous solutions containing soluble iron salts. Further, thethermotolerant bacteria culture used in the process of the presentinvention may be capable of oxidising iron (II) to iron (III) in acidicaqueous solutions and may be capable of oxidising reduced sulphurspecies to sulphate ion in acidic aqueous solutions.

Also, the thermotolerant bacteria culture used in the process of thepresent invention is preferably capable of oxidising iron and sulphidesin an aqueous liquid containing up to 20 grams/liter of sodium chloridewithout the addition of special nutrients or employment of the specialconditions. Thus, in this case extracting the pH, temperature, oxygen,nitrogen phosphate and potassium levels are maintained as discussedabove, oxidation will proceed.

Typically, a particular culture of thermotolerant bacteria contains oneor more bacteria species.

The process of the present invention can be operated in heaps, dumps,agitated systems or dams.

After completion of the oxidation step the oxidised solid residue andthe aqueous liquid are typically separated. In the case of preciousmetal recovery, the oxidised solid residue would preferably be washedand then the pH of the oxidised solid residue adjusted to a levelcompatible with the use of a cyanide leaching agent. Alternatively,another reagent such as thiourea could be used under acidic conditionsand so the need to adjust the pH is obviated.

EXAMPLES

The present invention will now be illustrated by the following examples.

EXAMPLE 1

A pyrite-gold concentrate designated P 1 was treated in accordance withthe present invention. The concentrate contained pyrite as the majorsulphide mineral with minor amounts of chalcopyrite, sphalerite, galenaand arsenopyrite. Other minerals present were quartz, sericite andsiderite.

The concentrate had the following assay.

                  TABLE 1                                                         ______________________________________                                        Assay of Pyrite Concentrate P 1                                               Element       Symbol   Assay (by weight)                                      ______________________________________                                        Gold          Au       52.0       ppm                                         Iron          Fe       26.0%                                                  Sulphur       S        27.5%                                                  Nickel        Ni       113        ppm                                         Copper        Cu       880        ppm                                         Zinc          Zn       320        ppm                                         Lead          Pb       160        ppm                                         Arsenic       As       3750       ppm                                         Silver        Ag       8          ppm                                         ______________________________________                                    

Samples of the concentrate were mixed with a sulphuric acid solution ata pulp density of 3% w/w to provide a pH range of 1.2 to 1.5. Nutrientsincluded in the acid solution were ammonium sulphate at 200 mg/L,di-potassium hydrogen phosphate at 200 mg/L and magnesium sulphateheptahydrate at 400 mg/L.

The acid level (pH) may vary from the start value and may either riseand then fall or fall from the outset. In most tests, the variation canbe significant with the final pH often less than 1.0.

The slurry was inoculated with a thermotolerant bacteria culturedesignated MTC 1. The inoculated slurry was shaken in conical flasks ata temperature of 43° C. Samples were removed periodically and analysedfor iron and arsenic extraction to determine the progress of thetreatment. The sample was treated by bacterial oxidation for 30 days toachieve 80% oxidation of the pyrite mineral. The solids weight loss dueto the oxidation process was 52%. The solid residue was then separatedfrom the residual acid solution. Leaching of the solid residue usingalkaline cyanide solution recovered 92% of the gold. In comparison,cyanide leaching could recover only 74% of the gold from the concentratein the untreated state. These results are summarised in Table 2.

                  TABLE 2                                                         ______________________________________                                        Gold Recovery from Untreated and Oxidised Concentrate                                                 Gold Recovered                                                                By Cyanide                                                        Iron Extracted                                                                            Leaching (by                                          Sample      (by weight) weight)                                               ______________________________________                                        Untreated    0%         74%                                                   Bacterial   80%         92%                                                   Oxidation                                                                     ______________________________________                                    

The cyanide solution employed to recover the gold contained sodiumcyanide at a concentration of 2 g/L. The iron in the solution from thebacterial oxidation process can be removed by adjusting the pH to above5.0 by the addition of lime, limestone, alkaline tailings or sodiumhydroxide.

The results of this test show that gold encapsulated with pyrite (FeS)can be released from the sulphide lattice by at least partial oxidationof the sulphur and iron by the thermotolerant bacteria culture MTC 1 torender the gold accessible to cyanide solution.

EXAMPLE 2

A nickel sulphide ore designated N 1 was treated in accordance with thepresent invention. The ore contained both sulphidic nickel andnon-sulphidic nickel minerals including violarite, lizardite andniccolite (NiAs) . Approximately 70% of the nickel was present assulphidic nickel. Other minerals were siderite, goethite, pyrite,chlorite and quartz.

The ore had the following assay.

                  TABLE 3                                                         ______________________________________                                        Assay of Nickel Ore N 1                                                       Element      Symbol   Assay (by weight)                                       ______________________________________                                        Nickel       Ni       2.74%                                                   Iron         Fe       18.7%                                                   ______________________________________                                    

Samples of the ore were mixed with a sulphuric acid solution at a pulpdensity of 13% w/w to provide a pH range of 1.2 to 1.5. Nutrientsincluded in the acid solution were ammonium sulphate am 200 mg/L,di-potassium hydrogen phosphate at 200 mg/L and magnesium sulphateheptahydrate at 400 mg/L.

The acid level (pH) may vary from the start value and may either riseand then fall or fall from the outset. In most tests, the variation canbe significant with the final pH often less than 1.0.

The slurry was inoculated with the thermotolerant bacteria culturedesignated MTC 1. The inoculated slurry was shaken in conical flasks ata temperature of 47° C. Samples were removed periodically and analysedfor iron and nickel extraction to determine the progress of thetreatment. At the completion of the bacterial oxidation treatment, 17days, the solution was removed from the residual solids and the residualsolids washed with sulphuric acid solution to remove any residualnickel. The nickel recovery was 93% after the residual nickel was washedout of the solids residue.

The nickel could be recovered from the solution by raising the pH to avalue of about 8.5, by the addition of lime or sodium hydroxide.

For comparison, the ore was also treated with iron (III) sulphatesolution at pH 1.0 .and 50° C. for 24 hours to extract nickel. Only 16%of the nickel was recovered in this process. These results aresummarized in Table 4.

                  TABLE 4                                                         ______________________________________                                        Nickel Recovery from Ore N 1                                                                 Nickel in Nickel                                               Treatment      Residue   Extraction                                           Method         (by weight)                                                                             (by weight)                                          ______________________________________                                        Iron (III)     2.03%     16%                                                  Leaching                                                                      Bacterial      0.60%     78%                                                  Oxidation                                                                     Bacterial      0.19%     93%                                                  Oxidation                                                                     & Washing                                                                     ______________________________________                                    

The results of this test showed that base metals in ore as sulphideminerals can be recovered by the action of the thermotolerant bacteriaculture MTC 1. The sulphidic minerals were oxidised to release thenickel into the acidic solution for conventional recovery.

EXAMPLE 3

A gold bearing arsenopyrite-pyrite concentrate was treated according tothe present invention, This concentrate was designated AP 1. The majorsulphide minerals were pyrite, 30% by weight and arsenopyrite, 35% byweight. Other minerals present were calcite, quartz and chlorite. Thegold was present almost completely in the arsenopyrite.

The concentrate had the following assay.

                  TABLE 5                                                         ______________________________________                                        Assay of Arsenopyrite Concentrate AP 1                                        Element       Symbol   Assay (by weight)                                      ______________________________________                                        Gold          Au       80         ppm                                         Arsenic       As       16.7%                                                  Iron          Fe       28.1%                                                  Sulphur       S        30.0%                                                  Nickel        Ni       1.5%                                                   ______________________________________                                    

Samples of the concentrate were mixed with a sulphuric acid solution ata pulp density of 3% w/w to provide a pH range of 1.0 to 1.3. Nutrientsincluded in the acid solution were ammonium sulphate at 200 mg/L,di-potassium hydrogen phosphate at 400 mg/L and magnesium sulphateheptahydrate at 400 mg/L.

The acid level (pH) may vary from the start value and may either riseand then fall or fall from the outset. In most tests, the variation canbe significant with the final pH often less than 1.0.

The slurry was inoculated with the thermotolerant bacteria culturedesignated MTC 1. The inoculate slurry was shaken in conical flasks at atemperature of 40° C. Samples were removed periodically and analysed foriron and arsenic extraction to determine the progress of the treatment.The sample was treated by bacterial oxidation for 12 days to achieve 90%break down of the arsenopyrite mineral. The solids weight loss due tothe oxidation process was 30%. The residual solids were then separatedfrom the acid solution. Leaching of the separated solid residue usingalkaline cyanide solution recovered 95% of the gold. In comparison,cyanide leaching could recover only 21% of the gold from the concentratein the untreated state. These results are summarised in Table 6.

                  TABLE 6                                                         ______________________________________                                        Gold Recovery from Untreated and Oxidised Concentrate                                                  Gold Recovered                                                      Arsenic   by Cyanide                                                          Extracted Leaching (by                                         Sample         (by weight)                                                                             weight)                                              ______________________________________                                        Untreated       0%       21                                                   Bacterial Oxidation                                                                          90%       95                                                   ______________________________________                                    

The cyanide solution employed to recover the gold contained sodiumcyanide at a concentration of 2 g/L. The arsenic and iron in thesolution from the bacterial oxidation process can be removed byadjusting the pH to above 5.0 by the addition of lime, limestone,alkaline tailings or sodium hydroxide.

The results of this test show that gold encapsulated with arsenopyrite(FeAsS) can be released from the sulphide lattice by at least partialoxidation of the arsenic, sulphur and iron by the thermotolerantbacteria culture MTC 1 to render the gold accessible to cyanidesolution.

EXAMPLE 4

A gold bearing arsenopyrite-pyrite concentrate was treated according tothe present invention. This concentrate was designated AP 2. The majorsulphide minerals were pyrite, 90% by weight and arsenopyrite 9% byweight. Other minerals present were calcite, quartz and chlorite. Thegold was distributed in both the arsenopyrite and the pyrite.

The concentrate-had the following assay.

                  TABLE 7                                                         ______________________________________                                        Assay of Arsenopyrite - Pyrite Concentrate AP 2                               Element       Symbol   Assay (by weight)                                      ______________________________________                                        Gold          Au       54         ppm                                         Arsenic       As       4.2%                                                   Iron          Fe       35.7%                                                  Sulphur       S        40.0%                                                  ______________________________________                                    

Samples of the concentrate were mixed with a sulphuric acid solution ata pulp density of 10% w/w to provide a pH range of 1.0 to 1.3. Nutrientsincluded in the acid solution were ammonium sulphate at 200 mg/L,di-potassium hydrogen phosphate at 400 mg/L and magnesium sulphateheptahydrate at 400 mg/L.

The acid level (pH) may vary from the start value and may either riseand then fall or fall from the outset. In most tests, the variation canbe significant with the final pH often less than 1.0.

The slurry was inoculated with the thermotolerant bacteria culturedesignated MTC 1. The inoculated slurry was shaken in conical flasks ata temperature of 53° C. Samples were removed periodically and analysedfor iron and arsenic extraction to determine the progress of thetreatment. The sample was treated by bacterial oxidation for 12 days toachieve 90% oxidation of the arsenopyrite mineral and an additional 21days for 70% pyrite oxidation as well as arsenopyrite oxidation. Theweight loss due to the oxidation process was 25% for the arsenopyriteand 78% for the 100% arsenopyrite plus 70% pyrite. The solids residuewas then separated from the acid solution.

Leaching of the solid residue using alkaline cyanide solution recovered79% of gold for the oxidation of 90% arsenopyrite and 87% for completeoxidation of the arsenopyrite and 70% of the pyrite in comparison,cyanide leaching could recover only 53% of the gold from the concentratein the uncreated state. These results are summarised in Table 8.

                  TABLE 8                                                         ______________________________________                                        Gold Recovery from Untreated and Oxidised Concentrate                                     Arsenic   Iron      %                                                         Extracted Extracted Gold Recovered                                Sample      (by weight)                                                                             (by weight)                                                                             (by weight)                                   ______________________________________                                        Untreated    0%        0%       53%                                           Bacterial Oxidation                                                                       90%       25%       79%                                           Bacterial Oxidation                                                                       100%      70%       87%                                           ______________________________________                                    

The cyanide solution employed to recover the gold contained sodiumcyanide at a concentration of 2 g/L.

The arsenic and iron in the solution from the bacterial oxidationprocess can be removed by adjusting the pH to above 5.0 by the additionof lime, limestone, alkaline tailings or sodium hydroxide.

The results of this test show that gold encapsulated with arsenopyrite(FeAsS) and in pyrite (FeS2) can be released from the sulphide latticeby at least partial oxidation of The arsenic, sulphur and iron by thethermotolerant bacteria culture MTC 1 to render the gold accessiblecyanide solution. This example also shows that the MTC 1 culture is ableto operate according to the invention at 53° C.

The thermotolerant bacteria culture MTC 1 was isolated from a coal minein Western Australia. Sludge and water samples were taken and used toinoculate volumes of a modified 9K medium containing yeast extract. Thesamples were incubated at 30° C., growth was observed after 7 days.These samples were then sub cultured in modified 9K medium without yeastextract.

Modifications and variations such as would be apparent to a skilledaddressee are deemed within the scope of the present invention.

We claim:
 1. A process for recovering metals from particulate refractoryprecious or base metal containing sulphide material comprising the stepsof contacting the sulphide material with an aqueous solution at atemperature in the range from 25° to 55° C., the aqueous solutioncontaining a thermotolerant bacteria culture having an optimum growthtemperature of 40° to 45° C. and capable of promoting oxidation of thesulphide material at a temperature in the range from 25° to 55° C.,separating the oxidized residue from the aqueous liquid and treating asubstance selected from a group consisting of the oxidized residue andthe aqueous liquid, to recover metal therefrom.
 2. A processingaccording to claim 1, wherein the aqueous solution is acidic.
 3. Aprocess according to claim 2, wherein the aqueous solution has a pH inthe range from 0.5 to 2.5.
 4. A process according to claim 1, whereinthe thermotolerant bacteria is acidophilic.
 5. A process according toclaim 1, wherein the thermotolerant bacteria is aerobic.
 6. A processaccording to claim 5, wherein the aqueous liquid is aerated during theoperation of the process.
 7. A process according to claim 1, wherein thethermotolerant bacteria is capable of autotrophic growth.
 8. A processaccording to claim 1, wherein no CO₂ is supplied to the thermotolerantbacteria during the operation of the process other than that availablefrom ambient air.
 9. A process according to claim 1, wherein the aqueousliquid contains sodium chloride in an amount up to 20 grams per liter.